NgForTrackByProperty

Angular global trackBy property directive with strict type checking.

This library is for templates written with *ngFor. Angular's built-in @for block has its own mandatory track expression, so code already migrated to the control flow syntax does not need it.

Description

When displaying a list of data (at least somewhat large lists) you should be using Angular's trackBy feature which looks something like:

import { Component } from '@angular/core';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  template: `
    <ul>
      <li *ngFor="let item of list; trackBy: trackById">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppListComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];

  public trackById(index: number, item: Item) {
    return item.id;
  }
}

Unfortunately, Angular forces us to write a tracking function in each component in which we want to make use of trackBy. With ng-for-track-by-property you could just handle this entirely in the template by passing a property like this:

import { Component } from '@angular/core';
import { NgForTrackByPropertyDirective } from 'ng-for-track-by-property';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  imports: [NgForTrackByPropertyDirective],
  template: `
    <ul>
      <li *ngFor="let item of list; trackByProperty: 'id'">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppListComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];
}

ng-for-track-by-property has strict type checking and only available property are allowed

See the stackblitz demo, or its sources.

Features

✅ trackBy property name
✅ Type casting
✅ trackBy index

Get Started

Step 1: install ng-for-track-by-property

npm i ng-for-track-by-property

Step 2: add trackByProperty to your ngFor, eg.:

import { Component } from '@angular/core';
import { NgForTrackByPropertyDirective } from 'ng-for-track-by-property';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  imports: [NgForTrackByPropertyDirective],
  template: `
    <ul>
      <li *ngFor="let item of list; trackByProperty: 'id'">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];
}

you can also track by index with trackByIndex, eg.:

import { Component } from '@angular/core';
import { NgForTrackByIndexDirective } from 'ng-for-track-by-property';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  imports: [NgForTrackByIndexDirective],
  template: `
    <ul>
      <li *ngFor="let item of list; trackByIndex">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];
}

since track by property id is a very common case, there is also trackById:

import { Component } from '@angular/core';
import { NgForTrackByIdDirective } from 'ng-for-track-by-property';

interface Item { 
  id: number; 
  name: string;
}

@Component({
  selector: 'app-root',
  imports: [NgForTrackByIdDirective],
  template: `
    <ul>
      <li *ngFor="let item of list; trackById">
        {{ item.id }} {{ item.name }}
      </li>
    </ul>
  `,
})
export class AppComponent {
  public list: Array<Item> = [
    { id: 0, name: 'foo' },
    { id: 1, name: 'bar' },
    { id: 2, name: 'baz' },
  ];
}

Available directives

Directive	HTML template syntax	TrackBy function
trackByProperty: key	*ngFor="let item of arr; trackByProperty: 'id'"	(index, item) => item[key]
trackByIndex	*ngFor="let item of arr; trackByIndex"	(index, item) => index
trackById	*ngFor="let item of arr; trackById"	(index, item) => item.id

Support

This is an open-source project. Star this repository, if you like it, or even donate. Thank you so much!

My other libraries

I have published some other Angular libraries, take a look:

• NgSimpleState: Simple state management in Angular with only Services and RxJS or Signal
• NgHttpCaching: Cache for HTTP requests in Angular application
• NgGenericPipe: Generic pipe for Angular application for use a component method into component template.
• NgLet: Structural directive for sharing data as local variable into html component template